Nondestructive identification of delaminations in concrete floor toppings with acoustic methods

This paper presents an original methodology for the nondestructive identification of delaminations in concrete floor toppings by means of the combined impulse-response and impact-echo acoustic methods. It is demonstrated that the impulse-response method is highly suitable for the fast exploration of large stretches of concrete floor and rough location of defective areas while the impact-echo method is ideal for the precise location of the boundaries of the areas. If the surface area of the tested floor topping is large, the nondestructive tests can be automated by mounting the equipment on a special scanner or robot. An example of the practical use of the proposed methodology is presented. It confirms the usefulness of the methodology for the nondestructive identification of delaminations in large-area concrete floor toppings.     

Identification of elastic stiffness and local stresses in concrete dams by in situ tests and neural networks

In this paper an experimental?–?numerical methodology is proposed for the in situ assessment of possibly deteriorated elastic properties of dam concrete and for the estimation of the local stress state in a concrete dam. The methodology described herein consists of the following operative stages: excavation of two parallel small holes (instead of the single one generally employed for rock testing); measurements of diameter variations in both holes by dilatometers; combination of experimental tests with computer simulations and inverse analyses for the parameter identification by means of artificial neural networks. Numerical validation tests of this parameter identification methodology are presented and its novelties and potentialities are discussed.     

Predicting moisture movement during the drying of concrete floors using finite elements

Predictive methods for determining the point at which it is safe to apply floor coverings to concrete floors can save time and money for the client, contractor and the floor installer. The current standard states that impervious floor coverings should not be applied until the surface of the floor reaches a relative humidity (RH) of 75%, established using a surface hygrometer test. Tests by the authors on drying concrete slabs in natural and forced drying environments show that there is a large variation in the residue of moisture deep in the concrete when the floor covering is applied, particularly so in a forced drying environment. After an impermeable floor covering is applied to the floor surface, this residue of moisture will gradually equilibrate within the slab depth and generate (over a long time) a vapour pressure at the surface that can result in substantial damage to the covering, resulting in expensive repair work.This paper presents a finite element model that predicts the changing moisture content, in terms of the internal RH, during drying and after the application of the floor covering as the internal RH equilibrates over time. The model accounts for the thickness of the slab, w/c ratio, environmental conditions, boundary conditions. It uses non-linear diffusion coefficients and evaporation rates to accurately model the moisture movement in the slab. The results from the model give good correlations with the experimental readings taken at the various depths over time using hand-held humidity probes.     

考虑施工需求的混凝土结构耐久性定量设计参数协调取值方法

由传统耐久性设计方法所确定的混凝土结构一维和二维扩散区的耐久性定量设计参数取值往往互不相同,导致混凝土的制备和施工浇筑存在困难。鉴于此,提出了考虑施工需求的混凝土结构耐久性定量设计参数的协调取值方法。首先考虑氯离子扩散系数的时变特性,推导了混凝土结构中一维和二维扩散区氯离子浓度分布的时变解析模型;然后分析了混凝土结构角部箍筋弯折对混凝土附加保护层厚度的影响,并依据不同扩散区耐久性定量设计参数的协调一致性要求,建立了箍筋协调弯弧内半径的计算表达式,进而提出了混凝土结构耐久性定量设计参数的协调取值方法,从而满足对于同一截面上具有不同扩散区的构件能够一次性完成混凝土浇筑的施工需求。分析表明,该方法基于箍筋协调弯弧内半径和耐久性定量设计确定混凝土结构的耐久性设计参数取值,使混凝土结构能够在规定的腐蚀环境作用下满足预定的设计使用年限要求,从而克服传统耐久性设计方法所存在的缺陷。     

An Unmanned Aerial Vehicle‐Based Imaging System for 3D Measurement of Unpaved Road Surface Distresses1

Abstract: Road condition data are important in transportation management systems. Over the last decades, significant progress has been made and new approaches have been proposed for efficient collection of pavement condition data. However, the assessment of unpaved road conditions has been rarely addressed in transportation research. Unpaved roads constitute approximately 40% of the U.S. road network, and are the lifeline in rural areas. Thus, it is important for timely identification and rectification of deformation on such roads. This article introduces an innovative Unmanned Aerial Vehicle (UAV)‐based digital imaging system focusing on efficient collection of surface condition data over rural roads. In contrast to other approaches, aerial assessment is proposed by exploring aerial imagery acquired from an unpiloted platform to derive a three‐dimensional (3D) surface model over a road distress area for distress measurement. The system consists of a low‐cost model helicopter equipped with a digital camera, a Global Positioning System (GPS) receiver and an Inertial Navigation System (INS), and a geomagnetic sensor. A set of image processing algorithms has been developed for precise orientation of the acquired images, and generation of 3D road surface models and orthoimages, which allows for accurate measurement of the size and the dimension of the road surface distresses. The developed system has been tested over several test sites with roads of various surface distresses. The experiments show that the system is capable for providing 3D information of surface distresses for road condition assessment. Experiment results demonstrate that the system is very promising and provides high accuracy and reliable results. Evaluation of the system using 2D and 3D models with known dimensions shows that subcentimeter measurement accuracy is readily achieved. The comparison of the derived 3D information with the onsite manual measurements of the road distresses reveals differences of 0.50 cm, demonstrating the potential of the presented system for future practice.     

Non-destructive identification of pull-off adhesion between concrete layers

This paper presents a methodology for the non-destructive identification of the values of pull-off adhesion between concrete layers in floors on the basis of parameters determined using three non-destructive testing (NDT) methods and artificial neural networks (ANN). The methodology is based on the earlier research by the author. There are three stages in the methodology: stage 1 in which two parameters are determined on the concrete substrate layer surface using the non-destructive 3D laser scanning method and three parameters are determined on the added concrete surface using the acoustic impulse response and impact-echo methods, stage 2 in which an ANN is trained and tested, and stage 3 in which numerical analyses of the results are carried out and the values of pull-off adhesion f_c,b are identified. It is shown that the methodology is practicable, as demonstrated by the provided example.     

Image‐based crack assessment of bridge piers using unmanned aerial vehicles and three‐dimensional scene reconstruction

Crack assessment of bridge piers using unmanned aerial vehicles (UAVs) eliminates unsafe factors of manual inspection and provides a potential way for the maintenance of transportation infrastructures. However, the implementation of UAV‐based crack assessment for real bridge piers is hindered by several key issues, including the following: (a) both perspective distortion and the geometry distortion by nonflat structural surfaces usually appear on crack images taken by the UAV system from the pier surface; however, these two kinds of distortions are difficult to correct at the same time; and (b) the crack image taken by a close‐range inspection flight UAV system is partially imaged, containing only a small part of the entire surface of the pier, and thereby hinders crack localization. In this paper, a new image‐based crack assessment methodology for bridge piers using UAV and three‐dimensional (3D) scene reconstruction is proposed. First, the data acquisition of UAV‐based crack assessment is discussed, and the UAV flight path and photography strategy for bridge pier assessment are proposed. Second, image‐based crack detection and 3D reconstruction are conducted to obtain crack width feature pair sequences and 3D surface models, respectively. Third, a new method of projecting cracks onto a meshed 3D surface triangular model is proposed, which can correct both the perspective distortion and geometry distortion by nonflat structural surfaces, and realize the crack localization. Field test investigations of crack assessment of a real bridge pier using a UAV are carried out for illustration, validation, and error analysis of the proposed methodology.     

Determination of chloride diffusivity through partially saturated Portland cement concrete by a simplified procedure

A recently proposed methodology for measuring the diffusion coefficient of chloride ions through concrete partially saturated with water, has been applied to five high-early-strength Portland cement mortars. The method is based on putting solid NaCl in contact with the concrete surface during the diffusion test. The results obtained indicate a strong dependence of the diffusion coefficient on the water saturation degree. The ionic diffusivities through these mortars are higher than those obtained with the same approach for five pozzolanic cement mortars of similar compositions. It has been also shown that the proposed test methodology and a different one based on the interaction of HCl(g) with the tested concrete surface, can yield comparable results. The D values obtained are of similar magnitude, and the same type of dependence on the water saturation degree is found through both methods, when applied to similar Portland cement concrete mixes with cement content of about 350 kg/m³ and water/cement ratios between 0.5 and 0.6.     

钢筋混凝土双向密肋夹芯保温叠合板研究

钢筋混凝土双向密肋夹芯保温叠合板由预制预应力混凝土底板、保温隔热夹芯和现浇混凝土面层组成.分析了预应力混凝土底板在施工阶段和使用阶段的受力状态,提出了钢筋混凝土双向密肋夹芯保温叠合板的设计计算方法,并对其施工工艺及质量检验提出了要求.     

Sand–concrete interface response: The role of surface texture and confinement conditions

Sand–concrete interface direct shear tests were used to investigate the effects of surface roughness, surface waviness, mean sand diameter and relative density on interface strength and behavior under different confinement conditions. Extreme concrete surface textures, including smooth, rough and rough–wavy textures, were reproduced. Surface plowing was assessed via image analysis, laser scanning and extended multifocal micrographs. The experimental results showed that smooth concrete surfaces exhibited high values of interfacial–to–internal friction angle ratios, ranging 88–90%, due to the angular shape of sand particles. The rough concrete surfaces generated higher interface strength than smooth concrete surfaces; however, the interface strength was still inferior to the surrounding sand strength. Surface plowing, which identified a mixed shear plane at the sand–concrete interface, was developed as particles were detached from the surface, thus inhibiting the interface friction angle from reaching the sand friction angle. Higher sand–concrete interface strength was achieved as surface waviness increased, and interface friction angles greater than the surrounding sand friction angle were reached. Under a constant normal stiffness condition, significantly high interface strength is achieved due to the increase of the current normal stress, which was directly influenced by the initial normal stress, stiffness, surface roughness, mean sand diameter and relative density; surface waviness did not have a marked effect on the normal stress variation. Based on these results, multiple regressions were proposed to estimate the sand–concrete interface strength by the interfacial–to–internal friction angle ratio and the effect of the constant normal stiffness condition.     

Shear crack pattern identification in concrete elements via distributed optical fibre grid

Abstract

An experimental methodology to obtain the shear cracking pattern in concrete elements is presented. The method is based on the use of Distributed Optical Fibre Sensors (DOFS) connected to an Optical Backscattered Reflectometer (OBR). Using this OBR system and a 2D grid conformed by one or two DOFS, the crack patterns of three partially pre-stressed concrete (PPC) beams subjected experimentally to shear failure, were obtained for increasing level of load. The 2D distributed fibre optic sensoring mesh was formed by attaching the fibre to the shear span of each beam using an epoxy adhesive. The importance of a correct DOFS bonding procedure to the concrete surface to obtain accurate results is described, and the principal advantages of DOFS to complement the use of discrete sensors in concrete experimental shear tests are shown. The proposed technique is a powerful tool to be implemented in the structural health monitoring in shear of concrete structures, where the variable inclined cracks are difficult to monitor by other experimental techniques using discrete sensors.     

Automating surface flatness control using terrestrial laser scanning and building information models

Current practice in the control of surface flatness requires a significant amount of time and labor, and delivers results based on few sample measurements. Developments of Terrestrial Laser Scanning (TLS) and Building Information Modeling (BIM) offer great opportunities to achieve a leap forward in the efficiency and completeness of dimensional control operations. This paper presents an approach that demonstrates the value of this integration for surface flatness control. The approach employs the Scan-vs-BIM principle of Bosché and Haas (2008) to segment TLS point clouds acquired on-site, by matching each point to the corresponding object in the BIM model. The novel approach then automatically applies two different standard flatness control techniques, Straightedge and F-Numbers, to the TLS points associated to each floor, and concludes with regard to their compliance with given tolerances. The approach is tested and validated using data from two actual concrete slabs. Results confirm the suitability of using TLS for conducting standard dimensional controls, and validate the performance of our system when compared to traditional measurement methods (in terms of both quality and efficiency). Furthermore, a novel straightedge generation method is proposed and demonstrated that enables more complete and homogeneous analysis of floor flatness for insignificant additional processing times.     

Analysis of Structures in Fire as Simplified Skeletal Frames Using a Customised Beam Finite Element

This paper presents a customised beam finite element and analysis formulation for structures in fire, where structures are modelled as skeletal frames, even for composite frames. This provides a simple and fast approach, suitable for design office analyses, with the objective of making structural fire design more accessible for consulting engineers. The methodology proposed has a generic formulation so is suitable for concrete, steel or composite structures. A single, resultant bending and axial stiffness is calculated for a generic cross-section. Stiffnesses are calculated about updated neutral axis positions, which can change during analyses. Thermal effects are applied using resultant pseudo forces. Three case studies are investigated with predicted and experimental deflections showing good correlation. Case Study 1 presents a concrete slab subjected to a standard fire, and deflections predicted by the proposed methodology are between 4.7% and 16.9% different from experimental results, comparable to an advanced modelling system. Two composite beams are considered in Case Study 2. Results for the first composite beam are almost indistinguishable from deflections predicted by a significantly more complicated 3D analysis method in the literature. The runaway failure that occurs during the second composite beam experiment is anticipated by the proposed formulation. In Case Study 3 a full composite floor slab subjected to a real fire is investigated. A novel contribution of the paper is demonstrated in that a slab experiencing tensile membrane action can be modelled using a series of beam elements. Predicted deflections are typically in the order of 12.7% different from experimental results (with both over and under predictions occurring), closer to experimental results than a contemporary full 3D analysis. The aforementioned case studies are carried out using subroutines coupled with a simple, commercially available, finite element program to demonstrate the effectiveness of the approach.     

Progressive collapse of multi-storey buildings due to failed floor impact

This paper proposes a new design-oriented methodology for progressive collapse assessment of floor systems within multi-storey buildings subject to impact from an above failed floor. The conceptual basis of the proposed framework is that the ability of the lower floor for arresting the falling floor depends on the amount of kinetic energy transmitted from the upper floor during impact. Three principal independent stages are employed in the proposed framework, including: (a) determination of the nonlinear static response of the impacted floor system, (b) dynamic assessment using a simplified energy balance approach, and (c) ductility assessment at the maximum level of dynamic deformation attained upon impact. In order to calibrate the proposed method, the part of the kinetic energy of the impacting floor that is transferred to the impacted floor is first theoretically determined for the two extreme impact possibilities, namely fully rigid and fully plastic impact. Moreover, a series of numerical studies is carried out to further refine the accuracy of this new approach with respect to different impact scenarios, whilst the effects of detailed joint modelling and redundancy are also investigated. The application of the proposed methodology is demonstrated by means of a case study, which considers the impact response of a floor plate within a typical multi-storey steel-framed composite building. Several possibilities regarding the location of the impacted floor plate, the nature of the impact event and the intensity of the gravity loads carried by the falling floor are examined. The application study illustrates the extremely onerous conditions imposed on the impacted floor resulting in an increased vulnerability to progressive collapse for structures of this type. Importantly, the likelihood of shear failure modes in addition to inadequate ductility supply under combined bending/axial actions is identified, thus establishing the need for further research work on the dynamic shear capacity of various connection types subject to extreme events.     

A state-of-the-art review of automated extraction of rock mass discontinuity characteristics using three-dimensional surface models

In the last two decades, significant research has been conducted in the field of automated extraction of rock mass discontinuity characteristics from three-dimensional (3D) models. This provides several methodologies for acquiring discontinuity measurements from 3D models, such as point clouds generated using laser scanning or photogrammetry. However, even with numerous automated and semi-automated methods presented in the literature, there is not one single method that can automatically characterize discontinuities accurately in a minimum of time. In this paper, we critically review all the existing methods proposed in the literature for the extraction of discontinuity characteristics such as joint sets and orientations, persistence, joint spacing, roughness and block size using point clouds, digital elevation maps, or meshes. As a result of this review, we identify the strengths and drawbacks of each method used for extracting those characteristics. We found that the approaches based on voxels and region growing are superior in extracting joint planes from 3D point clouds. Normal tensor voting with trace growth algorithm is a robust method for measuring joint trace length from 3D meshes. Spacing is estimated by calculating the perpendicular distance between joint planes. Several independent roughness indices are presented to quantify roughness from 3D surface models, but there is a need to incorporate these indices into automated methodologies. There is a lack of efficient algorithms for direct computation of block size from 3D rock mass surface models.     

Autonomous 3D vision-based bolt loosening assessment using micro aerial vehicles

Earlier identification of bolt loosening is crucial to maintain structural integrity and prevent system-level collapse. In this study, a novel drone-based 3D vision methodology has been proposed for autonomous bolt loosening assessment. First, a low-cost micro aerial vehicle with various types of sensors is designed. Second, a drone-based autonomous image collection method is proposed. Third, a 3D point cloud of the bolted connection is generated using the acquired images. Fourth, 3D point cloud processing methods are proposed to localize and quantify bolt loosening. The proposed method has been implemented on structural beam–column connections. The results show that the proposed drone-based data collection method can effectively acquire images for 3D reconstruction. The 3D point cloud processing methods can reliably localize and quantify bolt loosening at high accuracy. The proposed method provides a more robust and comprehensive evaluation of bolt loosening, compared to existing 2D vision methods, which process 2D images captured at a specific camera view.     

钢管束结构中混凝土楼板开裂分析

针对钢管混凝土束剪力墙结构(简称钢管束结构)中现浇混凝土楼板出现的开裂问题进行有限元分析研究,找出楼板开裂的原因并针对性地给出预防建议.根据钢管束结构不同于混凝土结构的特点,利用ANSYS建立了一套不同于传统设计方法的钢管束结构有限元建模方法,按照实际工程设计文件进行有限元建模分析.结果 表明:在正常使用阶段,楼板板面...     

Performance of a BOTDR optical fibre sensing technique for crack detection in concrete structures

Strain distribution measurements and problematic crack detection are important issues in the damage detection and performance evaluation of concrete, or reinforced concrete, structures. In recent years, the Brillouin Optical Time Domain Reflectometry (BOTDR) based optical fibre sensing technique has attracted great attention as a distributed monitoring method. Current BOTDR instruments are suitable for strain measurements over a certain distance (termed spatial resolution), but damage such as cracks in concrete structures are local. It is crucial to find an effective method to detect local damage in concrete. In this study, two basic optical fibre installation methods, overall bonding (OB) installation and point fixation (PF) installation, are proposed. Then, several unique installation methods (one-round, one-round superposition and two-round superposition) are proposed and investigated experimentally for a reinforced concrete bending beam. The efficiency of the proposed installation methods and the effect of the length of the sensing region on the measurement accuracy are also discussed. Experimental results show that the n-round superposition installation method can effectively and correctly detect the total crack width within a relatively local region. The performance of the overall bonding and point fixation installation methods with different sensing region lengths, or gauge lengths, for local crack initiation and total crack width measurement is also discussed.     

用预应力控制混凝土楼板非荷载裂缝的试验方法

采用现浇钢筋混凝土楼板的住宅,顶层经常发生非荷载裂缝。通过对现行规范及试验方法的分析,提出采用先张法对楼板部分受力钢筋施加预应力,可有效控制楼板的非荷载裂缝。介绍试验方法和装置,试验结果证明,这一技术能有效提高楼板的抗裂性能。     

砖混房屋现浇楼板裂缝原因分析及对策

长期以来 ,砖混结构现浇楼盖裂缝问题令许多工程技术人员感到棘手 ,且近年来日趋严重。结合一幢砖混结构宿舍楼现浇楼板裂缝实例 ,计算分析了该种结构现浇楼板常见裂缝产生的原因 ,并提出了相应的处理和预防措施。后续类似工程证明所建议的裂缝预防措拴是有效的。